The present invention discloses a diesel oxidation catalyst comprising a honeycomb substrate washcoated with a mixture of clay, a refractory oxide and a zeolite, upon which is further deposited a precious metal catalyst and a sulfur oxidation suppressant. In a preferred embodiment, the clay is bentonite, the refractory oxide is zirconia or titania, the zeolite is beta-zeolite or Y-zeolite or ZSM-5 or mordenite, the precious metal catalyst is preferably platinum, and the sulfur oxidation suppressant is vanadium or vanadium oxide or a combination thereof
Internal combustion engines function by burning fuels (hydrocarbons) at high temperatures. In theory, the products of the combustion process are CO2 and water. But, it is not uncommon that the combustion process is incomplete resulting in the formation of undesirable byproducts are formed such as carbon monoxide, hydrocarbons and soot. Other reactions occurring in internal combustion engines include the oxidation of nitrogen molecules to produce nitrogen oxides and the oxidation of sulfur to form SO2 and small percentage of SO3. Further, when the temperature decreases, the SO3 can react with H2O to form sulfuric acid. Other inorganic materials are formed as ash. The products of these reactions result in undesirable gaseous, liquid and solid emissions from internal combustion engine: gaseous emissions—carbon monoxide, hydrocarbons, nitrogen oxides, sulfur dioxide; liquid phase emissions—unburned fuel, lubricants, sulfuric acid; and, solid phase emissions—carbon (soot). The combination of liquid phase hydrocarbons, solid phase soot and sulfuric acid results in the formation of small size droplets often called total particulate matter. These emissions create pollution and are potential health risks.
Efforts have been made to develop exhaust gas cleaning catalysts for a number of years. U.S. Pat. No. 4,749,671 (issued to Saito et al., on Jun. 7, 1988) teaches an exhaust gas catalyst composition composed of a refractory three-dimensional structure with a catalytically active substance thereof The catalyst is designed to burn fine carbonaceous particles in an exhaust gas from an automobile engine, particularly a diesel engine, at lower temperatures. The refractory three-dimensional structure taught is a ceramic foam, an open-flow ceramic honeycomb, a wall-flow honeycomb monolithic body, a metal honeycomb or a metal foam. To obtain the desired exhaust gas cleaning, the refractory three-dimensional structure includes numerous irregularly arranged protrusions composed of refractory inorganic powder having a particle diameter of 5 to 300 micrometers or a mixture of it with refractory inorganic fibers and the catalytically active substance supported on the protrusions. The refractory inorganic powder taught is a powder of at least one material selected from the group consisting of active alumina, silica, titania, zirconia, silica-alumina, alumina-zirconia, alumina-titania, silica-titania, silica-zirconia, titania-zirconia and zeolite. In a claimed embodiment, the catalytically active substance is at least one of the group of platinum, palladium, and rhodium, and at least one element from the group of vanadium, iron, cobalt, nickel, molybdenum, tungsten, niobium, phosphorus, lead, zinc, tin, copper, chromium, manganese, cerium, lanthanum, silver, barium, magnesium, calcium, strontium, potassium, sodium, cesium and rubidium.
U.S. Pat. No. 5,628,975 (issued to Horiuchi et al., on May 13, 1997) teaches a method for purifying exhaust gases from a diesel engine by passing the exhaust gases through a honeycomb catalyst of specified geometry. The honeycomb catalyst comprises a flow-through metal or ceramic honeycomb carrier having 250 through cells per square inch parallel to the direction of flow of the exhaust gases and at least one catalytically active component deposited thereon selected from the group consisting of platinum, palladium, rhodium, iridium, vanadium, molybdenum, copper, silver, gold, rare earth elements and partially substituted perovskite composite oxides having an oxygen vacancy, and optionally, at least one refractory inorganic oxide selected from the group consisting of alumina, silica, titania, zirconia, and zeolite.
However, neither the '671 patent nor the '975 patent teach or suggest that an acid-leached bentonite can be used in the catalyst. Acid-leaching of the bentonite results in a clay that has greater than about a 90 wt % silica content, allowing for greater dispersion of the precious metals within the catalyst and, hence, better performance of the catalyst as compared to catalysts of the prior art.